Crash report

What happened?

It's possible to segfault or abort a no-gil interpreter running with PYTHON_GIL=0 by calling atexit functions from threads:

from threading import Thread
import atexit
def g():
    pass
def f():
    for x in range(100):
        atexit.register(g)
        atexit._clear()
        atexit.register(g)
for x in range(100):
    Thread(target=f, args=()).start()

Found using fusil by @vstinner.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a1+ experimental free-threading build (heads/main-dirty:612ac283b81, Nov 16 2024, 01:37:56) [GCC 11.4.0]

Linked PRs

• gh-126907: Lock the atexit state on the free-threaded build #126908
• gh-126907: Use a list for atexit callbacks #127935